Troutnut on Oct 19, 2009October 19th, 2009, 4:47 pm EDT
Yep: Hare's Ear, Pheasant Tail, and Prince would be the ones in the most fishermens' fly boxes, I think. Probably in that order, although PT might be first.
I was a little confused by the question, though: "most imitated" would be asking about the actual insects, but "in the fly boxes" would be asking about the flies. Everyone's answered the latter.
The former is harder; it depends how broad a category you want to look at. For a single species, it's probably the Hendrickson nymph. For a family, probably Baetidae (BWO) nymphs.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist